A conventional technology proposed, for instance, in JP-A-2006-22763 uses a turbocharger model to calculate boost pressure from exhaust characteristics. More specifically, this technology uses a turbine model to calculate turbine motive power from exhaust characteristics, and uses a shaft model to convert the turbine motive power to compressor motive power with a predetermined conversion efficiency. Further, a compressor model is used not only to calculate supercharging power which is actually used for supercharging, from the compressor motive power, but also to estimate the boost pressure from the supercharging power.